Blades et al., U.S. Pat. No. 3,081,519 (assigned to E.I. du Pont de Nemours and Company "DuPont")) describes a process wherein a solution of fiber-forming polymer in a liquid spin agent is flash-spun into a zone of lower temperature and substantially lower pressure to generate plexifilamentary film-fibril strands. Anderson et al., U.S. Pat. No. 3,227,794 (assigned to DuPont) discloses that plexifilamentary film-fibril strands are best obtained using the process disclosed in Blades et al. when, in a preflashing letdown chamber, the pressure of the polymer and spin agent solution is reduced so as to form a two-phase solution comprised of a fine homogeneous dispersion of a spin agent rich phase in a polymer rich phase. When this two-phase dispersion is released through a spinning orifice into a zone of lower temperature and pressure, the spin agent vaporizes and thereby cools the polymer which in turn forms the plexifilamentary strands.
The term "plexifilamentary strand", as used herein, means a strand which is characterized as a three-dimensional integral network of a multitude of thin, ribbon-like, film-fibril elements of random length and with a mean film thickness of less than about 4 microns and a median fiber width of less than about 25 microns, that are generally coextensively aligned with the longitudinal axis of the strand. In plexifilamentary strands, the film-fibril elements intermittently unite and separate at irregular intervals in various places throughout the length, width and thickness of the strand to form the three-dimensional network.
Anderson et al. discloses that successful flash-spinning of plexifilamentary strands according to the process of Blades et al. requires precise control of process parameters such as pressure, temperature and the ratio of polymer to spin agent. Solution flash-spinning of polymers according to the process of Blades et al. and Anderson et al. is restricted to those polymers for which there exists a compatible spin agent that: (1) is a non-solvent to the polymer below the spin agent's normal boiling point; (2) forms a solution with the polymer at high pressure; (3) forms a desired two-phase dispersion with the polymer when pressure is reduced slightly in a letdown chamber; and (4) flash vaporizes when released from the letdown chamber into a zone of substantially lower pressure. Solution flash-spinning his rarely been used to spin polymer blends because multiple polymers generally do not spin well from a single spin agent and under a single set of processing conditions.
European Patent Publication 645480 filed by Unitika Ltd. discloses a plexifilamentary fiber structure that is flash-spun from a solution of polyolefin and polyester polymers dissolved in methylene chloride. The polyolefins disclosed include polyethylene and polypropylene polymers and copolymers. The polyesters disclosed include polyethylene terephthalate and polybutylene terephthalate. The Unitika patent discloses that the mixing ratio (by weight) of the polyolefin to the polyester is from 5/95 to 95/5.
British Patent Specification 970,070 (assigned to DuPont) discloses nonwoven sheets made from fibers that were flash-spun from a blend of polyethylene and a minor amount of another polymer such as polyamide, polyvinyl chloride, polystyrene or polyurethane.
It has been found that quality plexifilamentary fiber strands can be spun from a finely divided dispersion of polymer in a spin agent without first forming a solution of the polymer and the spin agent. A process for flash-spinning of polymers from a mechanically generated dispersion of polymer, CO.sub.2 and water was disclosed in Coates et al., U.S. Pat. No. 5,192,468 (assigned to DuPont), which is hereby incorporated by reference. Among the polymers spun in Coates et al. are polyethylene blended with an ethylene vinyl alcohol copolymer, and polypropylene blended with an ethylene vinyl alcohol copolymer.
Blending incompatible polymers into a single fiber has historically led to some deterioration of properties, especially in the property of ultimate fiber strength. For example, recent work in melt spinning blends of polyethylene terepthalate (PET) and nylon 6 has shown that the addition of 5% of nylon 6 to PET results in a 5% loss in tenacity and break elongation (Journal of Applied Polymer Science, Vol. 55, pages 57-67 (1995)). Thus, it would not be expected that flash-spun blends of three or more incompatible polymers could actually improve fiber properties, including fiber tenacity.
It has now been discovered that blends of three or more polymers can be flash-spun, either from a mechanically generated dispersion of polymer, super critical carbon dioxide and water, or from a solution of a polymer in a solvent. It has also been found that the plexifilamentary strands spun from many such polymer blends have improved properties when compared to fibers flash-spun from just one or two of the polymers. The fiber strands of the invention will be useful in a variety of end uses, including filters, absorbent wipes, thermal and acoustical insulation materials, and garments.